The invention relates to an articulated device for the probe head of a coordinate measuring apparatus. The articulated device includes at least two rotational joints for angularly aligning the probe head.
Articulated devices have been known for some time. There is the so-called continuously rotatable articulated device wherein the rotational angle can be continuously adjusted via a corresponding motor and the exact rotational angle is supplied via an encoder. Also known are the latching articulated devices wherein only a limited number of rotational angles can be adjusted. In a known latching articulated device of Carl Zeiss having the product designation xe2x80x9cRDSxe2x80x9d, two interacting crown gears of a so-called Hirth meshing are used which, in the latched state, engage in one another and thereby latch the particular adjusted rotational angle.
A continuously rotatable articulated device is disclosed in U.S. Pat. No. 4,888,877. Here, an articulated device is shown which has two motor-driven rotational joints which are continuously pivotable and the rotational axes of which are perpendicular to each other. With reference to the different rotational angles, corrective values are stored so that the probe pin need not be newly calibrated for each change of the rotational angle of the rotational joints. The corrective values define the position of the rotational axes to each other. Furthermore, alternatively or in addition, corrective values can be provided with respect to angular position deviations and the running deviations of the rotational axes. However, with respect to the realization of these last-mentioned corrections, it is not precisely described how these corrections can be advantageously utilized.
The particular feature of the articulated device shown in U.S. Pat. No. 4,888,877, is that, for each individual error, a separate correction model is required so that, on the one hand, for several corrective models, the error parameters have to be determined as well as several corrective calculations have to be carried out for the correction of the measured measurement values. In the past, this has lead to the situation that, because of the high requirements as to a short measurement time, only the position of the rotational axes to each other and the angular position deviations of the rotational joints were corrected. These corrections, however, were applicable only for continuously rotatable articulated devices such as disclosed in U.S. Pat. No. 4,888,877. The corrective models were inadequate for latching articulated devices wherein the articulated device can be latched in a plurality of different positions. It is basically desirable to further improve measurement accuracy even for continuously rotatable articulated devices.
U.S. Pat. No. 5,138,563 discloses a correction method for an articulated device wherein errors are corrected which result because of the elastic deformation of the articulated device and the probe configuration. This is especially the case when the probe configuration includes a probe head extension on which the probe head is held. For the above purpose, the method, which is described in U.S. Pat. No. 5,138,563, was expanded for correcting the position of the rotational axes by a term {right arrow over (B)}(xcex1,xcex2) which indicates the deformation. The articulated device with the probe configuration attached thereto was viewed like the model of a bending beam so that, for determining the term {right arrow over (B)}(xcex1,xcex2), the maximum through bending of the articulated device was determined and then interpolated to the corresponding through bending in dependence upon the particular angle position of the rotational joint. The model has in the past provided good service. Because of the ever increasing requirements imposed on the measuring accuracy of coordinate measuring apparatus, the attempt was undertaken in U.S. Pat. No. 5,138,563 to configure the probe head extension, which is identified by the reference numeral 5 in this publication, to be stiffer. Here, it has, however, been shown that the correction results did not, as expected, become better because the deformations, which occurred in the articulated device, had become greater than the deformations in the probe head extension and therefore the basic model of the bending beam no longer functioned.
European patent publication 0,759,534 discloses a method for correcting a continuously rotatable articulating device or latching articulating device. In this publication, it is suggested to bring the articulating device into two different rotational angles and to make a calibration at each angle. A third rotational angle lying between the rotational angles is then corrected in that an interpolation is made between the recorded calibration data. Here, nothing is explained with respect to the correction model.
It is an object of the invention to provide an articulating device with which the bending, which is caused by the probe configuration, can be corrected in an improved manner.
The articulated device of the invention is for a probe head of a coordinate measuring apparatus and includes: a first rotational joint and a second rotational joint interconnected to angularly align the probe head with errors occurring because of an elastic deformation of the articulated device; and, a correction unit using a mathematical model therein incorporating at least one mathematical finite element to correct the errors caused by the elastic deformation when making a measurement.
The basic idea of the invention is that a mathematical model is used in the correction unit provided for correcting the deformation and that this mathematical model includes at least one finite element. In this way, the special advantage is afforded that the measuring results can be more precise with this feature. Such a correction model operates even precisely when the deformations, which occur in the articulating device, are greater than the deformations in the probe head extension so that a further reduction of the measuring inaccuracy can be achieved by the use of stiff probe head extensions.
The correction unit is a computer or microprocessor wherein the recorded corrective values are stored and which corrects the measured measurement values in correspondence to the selected mathematical model.
A mathematical finite element can be provided basically at any desired location and with a desired alignment in the articulating device. For reasons of computation, it is recommended that the finite element(s) be allocated to the rotational axes of the rotational joint and especially in such a manner that the elastic center of the finite element lies on a rotational axis of the joint.
The model should be so selected that the finite element can be mathematically displaced along the assigned axis in order to determine the location at which the best corrective results are obtained. In this way, it becomes also possible that errors can be considered, which are caused by a horizontally outwardly extending measuring arm of a stand-type measuring apparatus and by the connection of the articulating device to the measuring arm, in that the finite element can be mathematically displaced correspondingly in the direction toward the measuring arm.
A further simplification with respect to computation results when the finite element permits, as degrees of freedom, only rotations about the three axes of the coordinate system corresponding thereto.
Especially good results are obtained when an inherent finite element is assigned to each rotational axis.
It is here noted that the correction method can be used for latching articulating devices as well as for continuously rotatable articulating devices.